1. Field of the Invention
The present invention relates to an image reading apparatus using a plurality of sensor chips, an image reading method, and a program used therefor.
2. Description of the Related Art
In scanners for use by common users, contact image sensors (CISs) are used since they have advantages in that the required cost is reduced by the recent progress in the semiconductor process and production technology and in that they only need small amounts of luminous energy from light sources.
Recent scanners for use by common users have a main-scanning period of approximately 10 to 20 milliseconds, while image reading apparatuses, which are so-called “copying machines”, have such a two-digit increase in speed that the main-scanning period is 300 milliseconds.
When a CIS is used in the image reading apparatus having the double speed, one problem is that an increase in the temperature of sensor chips causes variation in a black offset level, which is a signal level for performing correction, to a reference level for the black signal by performing calculation on input signals and which can cancel (offsets) a difference in level between input signals. Specifically, in a mode of sequential reading of up to approximately fifty documents by using an auto document feeder (ADF), shading correction for offsetting an effective-image-signal level difference (e.g., a level difference between image signals in a line sensor when performing reading from a uniform-density reference member) is performed only once at the start of a job (e.g., an image-reading operation based on an operator's instruction). This is because mechanical implementation of shading correction for each document reduces copying productivity. Also, in order to meet the recent need to reduce power consumption, there is a case in which a CIS is not supplied with power when scanning is not performed, and by supplying power just before scanning, tens of documents are consecutively copied.
In this case, the temperature of a sensor chip and an analog chip rapidly rises for about the first one minute from a dead state and gradually increases thereafter, so that the black offset level changes. In addition to self-heating of the sensor chip and the analog chip, for example, when a Xenon lamp or the like is used as a document-illuminating light source, it is also a heat source and enhances the change.
As described above, the above-described related art has several problems. For example, it takes three minutes to sequentially read fifty documents set in the ADF from the start of reading to the end of reading, and the black offset level greatly changes between the reading of the first document and the reading of the fiftieth document.
This becomes particularly serious when a so-called “multi-chip sensor” formed by a plurality of arranged sensor chips is used. In other words, the change in black offset level between chips is not uniform compared with individual sensor-chip differences. Accordingly, a plurality of channels have different reference levels for a black signal. This generates brightness differences in image regions corresponding to the reference levels, so that visual image quality greatly deteriorates.